The in situ cleanup of soil and groundwater to remove contaminants has invited many attempts. The intention behind site cleanup is inherently green; however, remedial activities use energy, water, and material resources to achieve cleanup objectives. Traditional remediation technologies (e.g., pump and treat, air sparging, soil vapor extraction, or multiphase extraction) require electricity and fossil fuel to power equipment to remove contamination from soil and ground water. Extracted fluids are then processed aboveground, or disposed of in landfills when filters are used. The intractable nature of subsurface contamination suggests the need to explore the use of innovative technologies that reduce the environmental footprint of remedial treatments. Reactive materials in permeable reactive barriers (PRBs) have proven very useful for transforming or destroying organic waste in situ. Once emplaced they typically do not require a continued supply of electrical power and have the added benefit of creating a reactive zone for the destruction of contaminants in place.
Controlled-release techniques have been utilized extensively in diverse fields such as pharmaceutical and agrochemical technologies. However, controlled and sustained release of an oxidant during in situ chemical oxidation (ISCO) is an emerging concept that is extremely relevant to the field of environmental remediation, yet to-date has received little attention. ISCO using the oxidants permanganate, persulfate, and catalyzed hydrogen peroxide has shown great promise for remediation of many recalcitrant organic contaminants of concern (COC). Because the oxidant also reacts with natural organic matter, inorganic soil constituents, and other reduced compounds, the presence of a protective barrier that controls oxidant release may enhance the efficiency of ISCO and allow for long-term, low-cost treatment of chlorinated solvents. To this end, sustained release potassium permanganate and slow-release activated sodium persulfate were developed. Slow-release permanganate and activated persulfate have been demonstrated to be effective for sustained-removal of organic contaminants in both laboratory and field efforts (e.g., Dugan, et al., 2013; Christensen, et al., 2012; Khambu, et al., 2012). Paraffin wax was used as the environmentally benign and biodegradable matrix material for solid potassium permanganate (KMnO4) or sodium persulfate (NaS2O8) particles. The paraffin wax matrix protects the solid permanganate or persulfate particles from instant dissolution and facilitates slow-release of the oxidant over long periods of time. The sustained release oxidant materials contain between 65%-85% permanganate or persulfate and can be formed as cylinders for direct push applications in reactive barriers, or chipped material for hydro-fracturing into low permeability media.
The prior art has demonstrated that the permanganate concentrations will initially be high when flow is introduced to the system, however after short periods of time the oxidant concentrations decrease to unfavorable levels. As presented in FIG. 2, for example, permanganate concentrations may drop to a few hundred parts per million (or less) within a few months which results in incomplete degradation of organic contaminants due to decreased concentrations of the oxidant over time.
Coated oxidants have been proposed for use in in situ remediation of groundwater and ex situ treatment of water and wastewater. Wax coated potassium permanganate pellets, for example, have been suggested for injection into contaminated groundwater. It is suggested that the contaminants in the groundwater will dissolve the wax and that the thus exposed potassium permanganate will then oxidize the contaminants. U.S. Pat. No. 7,431,849. The potassium permanganate, or other oxidant, or combination of oxidants, in the pellets, is completely coated prior to contacting the contaminants in the groundwater. As provided in the prior art: “[W]hen the coated reactant is contacted with or exposed to the contaminants it dissolves, reacts, or absorbs at least one of the targeted compound(s) found in the media and exposes at least one reactant to the targeted compounds where it may react.” Id.
It has been hypothesized that wax coated permanganate when used to oxidize contaminated groundwater did so either by diffusion of the contaminant through the wax coating surrounding the potassium permanganate or by dissolution of the wax coating by the contaminant, thus exposing the potassium permanganate. “The Characteristics of Potassium Permanganate Coated in Polymer,” Ross, C. M., Thesis (M.S.), Clemson University (2001). The study sought to make completely coated permanganate.
Wax coated potassium permanganate cylinders have also been suggested for insertion into contaminated groundwater. “Using Slow-release Permanganate Candles to Remove TCE From a Low Permeable Aquifer at a Former Landfill,” Christenson, Mark D., et al., University of Nebraska-Lincoln, Oct. 1, 2012. The cylinders, also referred to as “candles,” are made by adding heated potassium permanganate to melted paraffin wax and then mixing the melted wax and potassium permanganate until all of the potassium permanganate is blended with the wax.
Combinations of solid oxidants coated in a dissolvable matrix, such as a wax matrix, have been proposed for use in in situ remediation of contaminated groundwater. U.S. Pat. No. 7,431,849. In this instance, a first oxidant is first coated with wax, then with a second oxidant, and the combination is then coated with wax. The resulting particle is multilayered with layers of oxidant separated by wax. Id.
Combinations of liquid oxidants have been used for in situ treatment of groundwater. It has been reported that a solution of potassium permanganate and a solution of sodium persulfate may be added simultaneously or serially to oxidize contaminants in situ in groundwater. U.S. Pat. No. 6,474,908. According to the prior art, the soil oxidant demand is satisfied by the sodium persulfate and volatile organic compounds are oxidized by the potassium permanganate. Id.
It appears that the prior art relies on either completely coated oxidants or liquid oxidants, whether a single oxidant or multiple oxidants are used for treatment of contaminated water and wastewater. The coated reactants of the prior art tend to release the coated oxidants at an unsteady rate which, in some instances, may be dependent on the time it takes for the groundwater contaminant to dissolve the encapsulant and expose the oxidant. If the dissolution of the encapsulant occurs slowly, then the oxidant concentration released from the coated reactant will be initially low with a sudden increase once dissolution of the encapsulant is well under way. If the encapsulant dissolves quickly, then the oxidant will be substantially released initially and then the concentration of oxidant will quickly diminish over time. When applied to the in situ treatment of contaminated groundwater, such prior art coated reactants may not be effective in substantially reducing the contaminant concentration because of the unsteady release of oxidants.
Accordingly, what is needed is a coated oxidant that provides for a steady or essentially uniform release of oxidants into contaminated water or wastewater. What is also needed is a coated oxidant that includes a combination of oxidants, such as potassium permanganate and sodium persulfate, and provides for a steady or essentially uniform release of oxidants into contaminated water or wastewater.